Antennas for many small satellites have been pretty simple monopoles set to spring out during the launch release. The antenna radiation from a monopole antenna using the satellite body as the counterpoise is primarily a linear polarization pattern in the general form of a toroid. A monopole antenna in this sense is considered to be a quarter wave whip while using the small satellite as the counterpoise. In order to receive the maximum signal from this linear monopole antenna pattern, a similarly polarized antenna at the ground station is desirable. Then the ground station antenna should be able to be rotated about the axis between the satellite and the ground station in order to keep receiving the maximum available signal level. The polarization changes for a linearly polarized antenna as it traverses its orbit. Thus, the relative polarization as received at the ground station antenna changes. The ground station antenna should be able to track this polarization rotation as well as to be able to track the movement of the small satellite.
Another aspect of the simple monopole antennas is that the linear antenna gain pattern can be maximum below the nadir. At slant angles approaching 60 degrees, the antenna gain is reduced more than 10 dB.
The small satellite named A0-73, FIG. 4, is representative of similar small satellites that use simple monopoles or dipoles for the communication antennas. The problem with these antennas is that their radiation pattern resembles the pattern shown in FIG. 6. The problem with these kinds of antennas is that they have a relatively narrow beamwidth. The narrow beamwidth reduces the time the satellite will have sufficient communication link margin. The reduced time for communication then limits the amount of data or communications to a ground station.
FIG. 4 shows a perspective view of prior art AO-73 Fun Cube Antennas. The AO-73 Fun Cube satellite is an example of simple antennas on a quite small satellite. While adequate for short communication, the antennas provide limited capability for communication. The subject invention, quadrifilar helical antenna can increase the coverage area and the amount of communication per orbit over a ground station.
FIG. 5 shows a prior art view of a 436.5 MHZ Horizontal Monopole on the right, and a 145 MHz Monopole on left side of a 1U small satellite with a wide beam along the X orbit direction and narrow beam on both sides of the X path, another example of prior art. The problem with these antennas is that the narrow beam widths reduce the time the satellite will have sufficient communication link margin. The reduced time for communication then limits the amount of data or communications to a ground station.
FIG. 6 is a perspective view of a prior art antenna pattern from the 436.5 MHz monopole from FIG. 5. For a satellite traveling in the +Y direction, there is a wide beam area along the X axis, but it is narrow in the Y direction. For a satellite traveling in +X direction, there is a narrow beam along the X axis, but narrow on either side of the X path. These narrow beam widths reduce the useable time and data available from the satellite as compared to an Iso-Flux Quadrifilar Helical Antenna in a deployed position.
The orbit of Small satellites is not well established soon after launch. It is difficult to find the small satellite when confined to utilizing azimuth, elevation rotatable Yagi's to locate the small satellite.
There are increasing numbers of small satellites also known as cube sats to be launched into mostly Low Earth Orbit from a dispenser being added to rockets that launch objects into space. These dispensers are becoming uniform amongst several rockets. As such, communication antennas are ideally stowed within the size restrictions of the dispenser.
The size of the small satellites is becoming known for the standard sizes as 1U through about 27U. A 1U cube satellite is approximately 100 mm×100 mm×100 mm. The realizable radiation patterns are less than optimal for the simple monopole antennas. Typical radiation patterns of 2 meter, 70 centimeters, and 12.5 centimeter antennas mounted on small satellites will be shown to provide illustrations of the diminished coverage area compared to a Quadrifilar Helical Antenna.
Other practitioners, even going back to Charles Kilgus, the original inventor of the quadrature filar antenna, focused their attention on the number of turns or partial turns of the structure; seemingly focused on incremental numbers of quarter wavelengths. Those antennas were mostly set to achieve a hemispheric antenna pattern.
U.S. Pat. No. 8,970,447 to Ochoa et al. describes a deployable helical antenna for nano-satellites. The Ochoa patent deployment means depends on use of a fuse-type element that when heated, breaks and allows the cover to flip open under a spring force. This means small pieces might break away into space which is undesirable. The helical antenna elements are wound outside of vertical stiffeners where upon release the elements are supposed to retain their desired shape. The patent loosely discusses folding and rolling the antenna into a stowed position with only a set of stiffeners that are supposed to resume a shape upon deployment. The stiffeners are suggested to be made from thermoplastic impregnated fiberglass. These stiffeners and their means to be attached to the helical metalized windings do not address the stresses after being stowed for long periods of time.
Further, the Ochoa patent is for dual wound helical antennas, not quadrifilar helical antennas. The Ochoa patent does not discuss any means to impedance match the antenna to a transmission line. The Ochoa patent does not discuss application to a quadrifilar helical antenna and therefore serves a different radiation pattern. The Ochoa patent does not discuss the possibility of shaping the antenna to develop an Iso-Flux antenna pattern.
Thus, there exists the need for solutions to the problems with the prior art.